Amperometric electrochemical sensors require a counter electrode to balance the current generated by the species being measured at the working electrode. In the case of a glucose oxidase based glucose sensor, the species being measured at the working electrode is H2O2. Glucose oxidase catalyzes the conversion of oxygen and glucose to hydrogen peroxide and gluconate according to the following reaction:Glucose+O2→Gluconate+H2O2 
Because for each glucose molecule metabolized, there is a proportional change in the product H2O2, one can monitor the change in H2O2 to determine glucose concentration. Oxidation of H2O2 by the working electrode is balanced by reduction of ambient oxygen, enzyme generated H2O2, or other reducible species at the counter electrode. In vivo glucose concentration may vary from about one hundred times or more that of the oxygen concentration. Consequently, oxygen becomes a limiting reactant in the electrochemical reaction and when insufficient oxygen is provided to the sensor, the sensor will be unable to accurately measure glucose concentration. Those skilled in the art have come to interpret oxygen limitations resulting in depressed function as being a problem of availability of oxygen to the enzyme.
As shown in FIG. 1, the sensor head 10 includes a working electrode 21 (anode), counter electrode 22 (cathode), and reference electrode 20 which are affixed to the head by both brazing 26 the electrode metal to the ceramic and potting with epoxy 28. The working electrode 21 (anode) and counter-electrode 22 (cathode) of a glucose oxidase-based glucose sensor head 10 require oxygen in different capacities. Prior art teaches an enzyme-containing membrane that resides above an amperometric electrochemical sensor. In FIG. 1, region 32 includes an immobilized enzyme, i.e. glucose oxidase. Within the enzyme layer above the working electrode 21, oxygen is required for the production of H2O2 from glucose. The H2O2 produced from the glucose oxidase reaction further reacts at surface 21a of working electrode 21 and produces two electrons. The products of this reaction are two protons (2H+), two electrons (2e−), and one oxygen molecule (O2) (Fraser, D. M. “An Introduction to In Vivo Biosensing: Progress and problems.” In “Biosensors and the Body,” D. M. Fraser, ed., 1997, pp. 1-56 John Wiley and Sons, New York). In theory, the oxygen concentration near the working electrode 21, which is consumed during the glucose oxidase reaction, is replenished by the second reaction at the working electrode. Therefore, the net consumption of oxygen is zero. In practice, neither all of the H2O2 produced by the enzyme diffuses to the working electrode surface nor does all of the oxygen produced at the electrode diffuse to the enzyme domain.
With further reference to FIG. 1, the counter electrode 22 utilizes oxygen as an electron acceptor. The most likely reducible species for this system are oxygen or enzyme generated peroxide (Fraser, D. M. supra). There are two main pathways by which oxygen may be consumed at the counter electrode 22. These are a four-electron pathway to produce hydroxide and a two-electron pathway to produce hydrogen peroxide. The two-electron pathway is shown in FIG. 1. Oxygen is further consumed above the counter electrode by the glucose oxidase in region 32. Due to the oxygen consumption by both the enzyme and the counter electrode, there is a net consumption of oxygen at the surface 22a of the counter electrode. Theoretically, in the domain of the working electrode there is significantly less net loss of oxygen than in the region of the counter electrode. In addition, there is a close correlation between the ability of the counter electrode to maintain current balance and sensor function. Taken together, it appears that counter electrode function becomes limited before the enzyme reaction becomes limited when oxygen concentration is lowered.
Those practicing in the field of implantable glucose oxidase sensors have focused on improving sensor function by increasing the local concentration of oxygen in the region of the working electrode. (Fraser, D. M. supra).
We have observed that in some cases, loss of glucose oxidase sensor function may not be due to a limitation of oxygen in the enzyme layer near the working electrode, but may instead be due to a limitation of oxygen at the counter electrode. In the presence of increasing glucose concentrations, a higher peroxide concentration results, thereby increasing the current at the working electrode. When this occurs, the counter electrode limitation begins to manifest itself as this electrode moves to increasingly negative voltages in the search for reducible species. When a sufficient supply of reducible species, such as oxygen, are not available, the counter electrode voltage reaches a circuitry limit of −0.6V resulting in compromised sensor function (see FIG. 3).
FIG. 3 shows simultaneous measurement of counter-electrode voltage and sensor output to glucose levels from a glucose sensor implanted subcutaneously in a canine host. It can be observed that as glucose levels increase, the counter electrode voltage decreases. When the counter electrode voltage reaches −0.6V, the signal to noise ratio increases significantly. This reduces the accuracy of the device. FIG. 4 shows a further example of another glucose sensor in which the counter-electrode reaches the circuitry limit. Again, once the counter electrode reaches −0.6V, the sensitivity and/or signal to noise ratio of the device is compromised. In both of these examples, glucose levels reached nearly 300 mg/dl. However, in FIG. 3 the sensor showed a greater than three-fold higher current output than the sensor in FIG. 4. These data suggest that there may be a limitation of reducible species at the counter electrode, which may limit the sensitivity of the device as the glucose levels increase. In contrast, FIG. 5 shows a glucose sensor in which the counter electrode voltage did not reach −0.6V. In FIG. 5 it can be observed that the sensor was able to maintain a current balance between the working and counter electrodes, thereby enabling accurate measurements throughout the course of the experiment. The results shown in FIGS. 3, 4 and 5 led the present inventors to postulate that by keeping the counter electrode from reaching the circuitry limit, one could maintain sensitivity and accuracy of the device.
Two approaches have been utilized by others to relieve the counter electrode limitation described above. The first approach involves the widening of the potential range over which the counter electrode can move in the negative direction to avoid reaching circuitry limitations. Unfortunately, this approach increases undesirable products that are produced at lower potentials. One such product, hydrogen, may form at the counter electrode, which may then diffuse back to the working electrode. This may contribute to additional current resulting in erroneously high glucose concentration readings. Additionally, at these increasingly negative potentials, the probability of passivating or poisoning the counter electrode greatly increases. This effectively reduces the counter electrode surface area requiring a higher current density at the remaining area to maintain current balance. Furthermore, increased current load increases the negative potentials eventually resulting in electrode failure.
The second approach is utilizing the metal case of the device as a counter electrode (see U.S. Pat. No. 4,671,288, Gough or U.S. Pat. No. 5,914,026, Blubaugh). This provides an initial excess in surface area which is expected to serve the current balancing needs of the device over its lifetime. However, when the counter electrode reaction is a reduction reaction, as in Blubaugh, the normally present metal oxide layer will be reduced to bare metal over time leaving the surface subject to corrosion, poisoning, and eventual cascade failure. This problem is magnified when considering the various constituents of the body fluid that the metal casing is exposed to during in vivo use. To date, there has been no demonstration of long-term performance of such a device with this counter electrode geometry.
Consequently, there is a need for a sensor that will provide accurate analyte measurements, that reduces the potential for cascade failure due to increasing negative potentials, corrosion and poisoning, and that will function effectively and efficiently in low oxygen concentration environments.